Method for producing lithium hexafluorophosphate

ABSTRACT

In order to provide a method for producing lithium hexafluorophosphate capable of producing lithium hexafluorophosphate of a higher purity than in the related art without the necessity for after-treatment for removal of impurities, a method is characterized by filtering lithium hexafluorophosphate coexisting with a solvent and then carrying out after-filtering drying in a gas atmosphere containing PF 5 .

TECHNICAL FIELD

[0001] The present invention relates to a method for producing lithiumhexafluorophosphate, and specifically relates to a method for producinglithium hexafluorophosphate useful as an electrolyte for lithiumsecondary batteries and a catalyst for organic synthesis.

BACKGROUND OF THE INVENTION

[0002] Conventionally, lithium hexafluorophosphate is produced by thefollowing steps:

[0003] 1) Lithium fluoride (LiF) is dissolved in hydrofluoric acidanhydride to give a solution.

[0004] 2) PF₅ gas is blown into this solution. This operation causes LiFto react with PF₅ to produce lithium hexafluorophosphate.

[0005] 3) The solution is cooled to deposit lithium hexafluorophosphate.

[0006] 4) The solution is filtered to separate the deposited lithiumhexafluorophosphate.

[0007] 5) The separated lithium hexafluorophosphate is then dried toyield lithium hexafluorophosphate as a product.

[0008] 6) Following this step, a product of lithium hexafluorophosphateis dissolved in an organic solvent, for example, for use as anelectrolyte for a lithium battery.

[0009] However, according to the related method for producing lithiumhexafluorophosphate, many kinds of impurities such as lithium fluorideand oxyfluoride acid compound are contained in hexafluorophosphate.

[0010] When lithium hexafluorophosphate containing such impurities isdissolved in an organic solvent for use as an electrolyte for a lithiumbattery, lithium fluoride contaminate is not soluble in the organicsolvent and the electrolyte therefore has to be filtered.

[0011] Examples of methods for removing these impurities are a methodwhere lithium hexafluorophosphate is dissolved in organic solvent toremove impurities using an ion exchange resin (Japanese PublishedUnexamined Patent Application No. 1984-87774) and a method whereneutralization treatment is carried out in an organic solvent(1984-81870.) However, these methods are complex in operation andprovide low productivity.

[0012] As described above, related methods for producing lithiumhexafluorophosphate require complex after-treatment. Even ifafter-treatment is performed, lithium hexafluorophosphate of high puritycannot be produced, which is problematic.

[0013] The present invention provides a method for producing lithiumhexafluorophosphate capable of yielding lithium hexafluorophosphate ofhigh purity without requiring after-treatment.

DISCLOSURE OF THE INVENTION

[0014] The method for producing lithium hexafluorophosphate according tothe present invention is characterized by filtering lithiumhexafluorophosphate coexisting with the solvent and then drying afterfiltering in a gas atmosphere containing PF₅.

[0015] Organic solvents that may be used as the solvent are solventsthat do not react with hydrofluoric acid anhydride or lithiumhexafluorophosphate.

[0016] A state where lithium hexafluorophosphate coexists with thesolvent is a state in which the following steps are completed.

[0017] 1) Lithium fluoride (LiF) is dissolved in hydrofluoric acidanhydride to give a solution.

[0018] 2) PF₅ gas is blown into this solution. This operation causes LiFand PF₅ to react to produce lithium hexafluorophosphate.

[0019] 3) The solution is cooled to deposit lithium hexafluorophosphate.

[0020] In the present invention, the step of filtering the solution isperformed in a gas atmosphere containing PF₅.

[0021] Gases that may be employed as the gas containing PF₅ (atmospheregas, hereafter) are gases that are prepared by diluting PF₅ with a gas(for example, HF gas, HCl gas, nitrogen gas, argon gas, helium gas,xenon gas, dried air, a gas that does not react with PF₅ or lithiumhexafluorophosphate.)

[0022] The concentration of PF₅ in the atmosphere gas is preferably 1 to50 mol %, with 1 to 20 mol % being more preferable, and 1 to 10 mol %being still more preferable. A concentration less than 1 mol % mayresult in the effects being insufficient. A concentration of over 50 mol% causes an increase in PF₅ consumption and is therefore expensive.

[0023] On the other hand, 0.003 to 0.3 kg of PF₅ is preferable for 1 kgof lithium hexafluorophosphate, with 0.003 to 0.03 kg of PF₅ being morepreferable. less than 0.003 kg of PF₅ may not cause effects that aresufficient. An amount over 0.3 kg causes the increase in PF₅ consumptionwhich is expensive.

[0024] Meanwhile, the concentration of impurities, particularlymoisture, contained in the atmosphere gas is preferably 100 ppb orsmaller, more preferably 10 ppb or smaller, and still more preferably 1ppb or smaller. Such control brings about a yield of lithiumhexafluorophosphate of high purity.

[0025] The pressure of thee Atmosphere gas may be normal pressure. Thetemperature may also be normal.

[0026]FIG. 1 shows a filtering apparatus used in the present invention.

[0027] Drying of lithium hexafluorophosphate separated as describedabove is carried out in the atmosphere gas as described above.

[0028] A transition time between filtering and drying is preferablyshort during which no exposure to an outside (air) atmosphere ispreferable.

[0029] That shown in FIG. 2 is preferably used as a drying apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a conceptual diagram of the filtering apparatus used inthe present invention.

[0031]FIG. 2 is a conceptual diagram of the drying apparatus used in thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0032] An Embodiment will hp described below with reference to acomparative example.

[0033] (Embodiment 1)

[0034] Hydrofluoric acid anhydride solution containing 22% lithiumhexafluorophosphate is cooled at −20° C. to deposit crystals of lithiumhexafluorophosphate.

[0035] Next, this slurry is transferred to a filtering apparatus (20 L(liter) volume) with inlet and outlet orifices for the atmosphere gas tobe filtered for 10 minutes in a flow of the atmosphere gas consisting of3 mol % of PF, and 97 mol % of nitrogen at a flow rate of 15 L/min,dried in the same atmosphere gas at a room temperature for 25 min, andfinally heated at 105° C. overnight.

[0036] The amount of the lithium hexafluorophosphate crystal yielded was11.3 kg and impurities were 50 ppm in HF and 70 ppm in LiF.

[0037] The amount of PF₅ used was 0.008 kg a 1 kg of lithiumhexafluorophosphate.

COMPARATIVE EXAMPLE 1

[0038] Hydrofluoric acid anhydride solution containing 22% lithiumhexafluorophosphate was cooled at −20° C. to deposit a crystal oflithium hexafluorophosphate.

[0039] Next, this slurry was transferred to the filtering apparatus (20L (liter) volume) with inlet and outlet orifices for the atmosphere gasto be filtered for 10 minutes flowing the atmosphere gas exclusivelyconsisting of nitrogen at a flow rate of 15 L/min, dried in theatmosphere gas at a room temperature for 25 min, and finally heated at105° C. overnight.

[0040] The amount of the lithium hexafluorophosphate crystal yielded was11.1 kg and impurities were 58 ppm in HF and 930 ppm in LiF.

[0041] The slurry is decomposed during filtering and drying resulting inan increase in LiF as an impurity.

[0042] (Embodiment 2)

[0043] Hydrofluoric acid anhydride solution containing 22% of lithiumhexafluorophosphate is cooled at −20° C. to deposit a crystal of lithiumhexafluorophosphate.

[0044] Next, this slurry is transferred to filtering apparatus (100 L(liter) volume) with inlet and outlet orifices for the atmosphere gas tobe filtered for 60 minutes flowing the atmosphere gas consisting of 15mol % of PF₅ and 85 mol % of nitrogen at a flow rate of 12 L/min, driedin the same atmosphere gas at the room temperature for 30 min, andfurther dried for 90 min in the flow of the atmosphere gas consisting of5 mol % of PF₅ and 95 mol % of nitrogen at a 35 L/min flow rate, andfinally heated at 105° C. overnight.

[0045] The amount of the lithium hexafluorophosphate crystal yielded was57 kg and impurities was 52 ppm in HF and 90 ppm in LiF.

[0046] The amount of PF₅ used is 0.022 kg per 1 kg of lithiumhexafluorophosphate.

COMPARATIVE EXAMPLE 2

[0047] Hydrofluoric acid anhydride solution containing 22% lithiumhexafluorophosphate is cooled at −20° C. to deposit a crystal of lithiumhexafluorophosphate.

[0048] Next, this slurry is transferred to filtering apparatus (100 L(liter) volume) with inlet and outlet orifices for the atmosphere gas tobe filtered for 60 minutes in the flow of the atmosphere gas exclusivelyconsisting of nitrogen at a flow rate of 12 L/min, dried in theatmosphere gas at a room temperature for 30 min, and finally heated at105° C. overnight.

[0049] The amount of the lithium hexafluorophosphate crystal yielded was54 kg and impurities were 54 ppm in HF and 1190 ppm in LiF.

[0050] The slurry was decomposed during filtering and drying resultingin an increase in LiF as an impurity.

INDUSTRIAL APPLICABILITY

[0051] According to the present invention without after-treatment,lithium hexafluorophosphate of a higher purity than that of conventionalcan be yielded.

What is claimed is:
 1. A method for producing lithiumhexafluorophosphate characterized in that the method is carried out byfiltering lithium hexafluorophosphate coexisting with a solvent andcarrying out post-filtering drying in a gas atmosphere containing PF₅.2. The method for producing lithium hexafluorophosphate according toclaim 1, wherein concentration of PF₅ in the gas ranges from 1 to 50%.3. The method for producing lithium hexafluorophosphate according toclaim 1 or 2, wherein PF₅ ranges from 0.003 to 0.3 kg for 1 kg oflithium hexafluorophosphate.